China is a country having lack of water resource with the characters of deficiency in amount and asymmetrical distribution in space time. Especially with the rapid progress of economy in China recently, the water pollution becomes graved day by day, which aggravates the contradiction in water resource. Statistically, in 1999, the total discharged wastewater was 40.1 billion tons including 19.7 billion tons of industrial wastewater and 20.4 billion tons of municipal wastewater. The total Chemical Oxygen Demand (COD) discharged in wastewater is 13.89 million tons including 6.92 million tons in industrial wastewater and 6.97 million tons in municipal wastewater. Recently, the amount of discharged wastewater has been increasing. The discharged wastewater contaminates the water and seriously hinders the sustainable development of society, economy and environment. One of primary ways to resolve the problems of lack in water resource and water pollution is wastewater treatment, that is to say, pollutants in municipal wastewater and industrial wastewater are removed to let the parameter of treated wastewater accord with the discharge or reuse standards by engineering methods.
The biological treatment method is most conventionally used, most economical and most efficient, which utilizes metabolism of microorganisms to remove the organic material and vegetal nutrition including nitrogen and phosphorus, and removes colloid grains by flocculation. It includes two treatment methods. One is anaerobic treatment method. The other is aerobic treatment method. The difference between them is that the degradation microorganisms of former are anaerobic and molecular oxygen is not needed during biodegradation, while the degradation microorganisms of later are aerobic and molecular oxygen is needed.
Though aerobic treatment method consumes more energy than anaerobic one, aerobic method is most conventionally used for wastewater treatment because it can more rapidly and completely remove the organic pollutants than anaerobic one. It is classified into activated sludge method and biomembrane method. Difference between them is that the aerobic microorganisms grow in suspension as sludge in wastewater in the former while in wads fixedly in the later. They both have merits and faults, and have long development history. Generally, activated sludge method is more efficient in organic pollutants removal, and its capital investment is smaller than the other. But its operation is more complex. So it is often used in the case of large amount of wastewater. While the biomembrane method is more fit for the impact load caused by variation of water quality and amount. Its operation is simple, so it is often used to treat the small amount of wastewater or micro-polluted water. These qualitative comparisons, however, is cursory. Factually, in engineering application, the comparison on merits and faults of aerobic treatment methods should be considered extensively, including factors of technology, economy and society, et al.
As far as activated sludge method is concerned, the comparison on merits and faults of aerobic treatment methods concentrates on reactors including their performance: 1. capability to supply bioreaction efficiency, 2. capability to sustain bioreaction system, and 3. extent of difficulty in management and operation.
Wastewater treatment bioreactor supplies a place for microorganisms to keep their metabolism and degrade pollutants. So, how to design is the key factor determining wastewater treatment effect.
At present, typical activated sludge reactors at home and abroad are traditional activated sludge reactors (including plug flow reactor and continuous stirred tank reactor), oxidation ditch, sequencing batch reactors (including SBR and its transformed reactors, such as CAST, MSBR, and UNITANK reactors, et al), and high performance compact reactor (HCR), et al.
Viewed from the idea of activated sludge system, the process of aerobic reaction can be divided into three steps, also called three steps method, including (aerobic) bioreaction, settle (separation of sludge and water) and sludge recycle. To keep three steps to run harmoniously is the abundant and necessary condition to sustain aerobic system. For example, FIG. 1 depicts aerobic bioreaction taking place in bioreactor. Since activated sludge grows in suspension, sludge and wastewater is in a state of mix and flow out of aerobic reactor together. The sludge must be separated with wastewater in water-sludge segregator, recycled to reactor, and treated water flows out of the treatment system. If there is no sludge recycle, microorganisms in the system must be washed away by water and the system will not be sustained because the time of life of sludge is much longer that that of hydraulic retention time in reactor.
According to the disposal forms of three steps method, prior activated sludge reactors can be classified into two kinds as follows:
One is space disposal of three steps method.
Its characters: Three steps are disposed in space form. They are divided in physical space and every step all has its relatively self-governed structure, but run in phase, that is to say, they keep consistent in time so as to realize the continuous run of system. The primary fault of this kind is large space occupation. The typical reactors are traditional activated sludge reactors (including plug flow reactor and continuous stirred tank reactor) and oxidation ditch, et al.
The other is time disposal of three steps method.
Its characters: Utilizing the same space, this method is divided into three steps by time disposal. It occupies small area with relatively concentrated and complex structures. Besides depending on aeration system and reliable equipments, operational reliability depends on perfect design of water decanter or effluent device and time control with PLC, et al. Operational requirement is high, and dependency is strong. Typical reactors are SBR, CASS, MSBR, UNITANK and their corresponding technologies, et al. Primary faults are that they occupy large space area and biodegradation is not ideal.